The purpose of this experiment was to study the efficiency of protective substances on the effects of cosmic radiation in space on low density lipoproteins. This environment induced modifications in LDL consisting of an increase of lipid peroxidation markers (hydroperoxides, thiobarbituric acid reactive substances). In contrast, apo B was not affected by cosmic radiation as shown by the stability of the trinitrobenzenesulfonic acid reactivity and the tryptophan content. Furthermore, oxidation of LDL was partially inhibited by the addition of cysteamine or / and probucol before the spaceflight experiment. The hydroperoxide formation was almost completely inhibited by cysteamine. It was concluded that antioxidants can exert a protective effect against peroxidative stress induced by the space environment.